Additional Supporting Information may be found in the online version of this article.

mus23760-sup-0001-suppfig1.tif433KSUPPLEMENTARY FIGURE S1. Normalized signal intensity in images of fresh muscle samples over a range of near-infrared excitation wavelengths (880–1,060 nm). An excitation wavelength of 960 nm produced the highest signal. Colors indicate the emission filters used (center wavelength, nm/full width at half maximum, nm).
mus23760-sup-0002-suppfig2.tif1197KSUPPLEMENTARY FIGURE S2. Subjects were seated with the left arm in a custom brace. Microendoscopy images in the extensor carpi radialis brevis (ECRB) were collected with the wrist in extension and flexion, while other joints remained fixed. The elbow was flexed to approximately 135°. The forearm was pronated such that the wrist flexion axis was horizontal. Radial/ulnar deviation was neutral. The fingers were extended. Subjects minimized motion relative to the microendoscope by keeping a spot from a laser pointer attached to the left shoulder within a small target approximately one meter away.
mus23760-sup-0003-suppfig3.tif79KSUPPLEMENTARY FIGURE S3. Ultrasound image showing location of endoscope insertion into the extensor carpi radialis brevis (ECRB). The microendoscope was positioned at a repeatable depth and proximal–distal location. The position was selected to image sarcomeres within fibers (red line) that were at low angles to the microendoscope bottom face. Tip depth was 1 cm below the skin surface. In 3 subjects, fascicle angles near the endoscope tip were measured relative to the horizontal in ultrasound images. Angles were 5.8±3.3°. Scale bar=5 mm.
mus23760-sup-0004-suppfig4.tif1499KSUPPLEMENTARY FIGURE S4. The microendoscope was inserted into the subject's muscle with a custom needle delivery system. (a) A stainless steel central stylet with guide tube was inserted into the muscle using the injector (injector not shown). (b) The central stylet was removed, and the guide tube was left in the muscle. (c) The tube was held in place with a custom clamp, and the microendoscope was placed in the guide tube.
mus23760-sup-0005-suppfig5.tif1420KSUPPLEMENTARY FIGURE S5. Microendoscopy images of sarcomeres in the extensor carpi radialis brevis (ECRB) from 6 additional subjects with the wrist extended (left column) and flexed (right column). Images in the same row are from the same subject. Bright regions in the image (pseudocolored blue) are myosin-containing A-bands. Scale bar=10 μm.

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